In general, an exhaust gas recirculation (EGR) system increases concentration of CO2 in intake air by recirculating a portion of exhaust gas to an intake system, thereby decreasing temperature of a combustion chamber, and thus reducing NOx.
An exhaust gas heat exchanger (normally referred to as an EGR cooler) for cooling the exhaust gas by using a coolant is used in the EGR system. Since the exhaust gas heat exchanger cools exhaust gas temperature from about 700° C. to 150˜200° C., it is required to have heat resistance. Further, the exhaust gas heat exchanger is required to be compact so as to be mounted to a vehicle, and to minimize pressure reduction for supplying a proper amount of EGR. Additionally, when the exhaust gas is condensed during heat exchange, sulphur oxides are included in condensed water due to sulphur in the exhaust gas, which causes the exhaust gas heat exchanger to be easily corroded, and thus the exhaust gas heat exchanger is required to be corrosion-resistant. Further, since mechanical loads occur due to pulsation of the exhaust gas, the exhaust gas heat exchanger is required to have a predetermined mechanical strength.
The exhaust gas heat exchanger includes: a laminated tube core in which a plurality of gas channels are laminated; an exhaust gas passage through which the exhaust gas passes in each of the gas channels; and a coolant passage provided between adjacent gas channels. Further, the gas channel of the exhaust gas heat exchanger is provided with a fin structure, that is, a wave fin plate therein that can increase heat exchange efficiency by inducing turbulence of fluid. The wave fin plate normally referred to as a wavy fin includes a plurality of wave fins, and each of the wave fins has a sine curve shape of a fixed pitch that has a ridge shape and a groove shape arranged in series in an entire length of each of the wave fins.
As shown above, the sine curve shape of the wave fin having the fixed pitch causes turbulence in fluid, that is, the exhaust gas that passes through a fluid passage having the wave fin, thereby increasing heat exchange efficiency of the exhaust gas heat exchanger. Meanwhile, although a performance and the gas pressure difference reduction of an EGR cooler required when developing a vehicle depend on an engine of the vehicle, improved performance (or efficiency), and a gas pressure difference reduction are required in any kind of engine. However, the wave fin plate that includes wave fins having a fixed pitch sine curve shape has difficulty in maintaining efficiency and reducing the gas pressure difference.